1. Field of the Invention
The present invention relates to a rare earth magnet and a method for manufacturing the same. More particularly, the present invention relates to a high-performance rare earth sintered magnet manufactured of rare earth alloy powder having a reduced oxygen content.
2. Description of the Related Art
An R—Fe—B rare earth magnet (R is at least one kind of element selected from the group consisting of yttrium (Y) and rare earth elements) is mainly composed of a major phase made of an R2Fe14B tetragonal compound, an R-rich phase including a rare earth element such as Nd in a large proportion, and a B-rich phase including boron (B) in a large proportion. The magnetic properties of an R—Fe—B rare earth magnet are improved by increasing the proportion of an R2Fe14B tetragonal compound as the major phase in the magnet.
At least a minimum amount of the R-rich phase is necessary for liquid-phase sintering which is a necessary process for forming sintered rare earth magnets. Since R reacts with oxygen to generate an oxide R2O3, R is partly consumed prior to the sintering. Therefore, to compensate for the amount consumed by the oxidation, an additional amount of R is conventionally required. The oxide R2O3 is generated more vigorously as the amount of oxygen is greater. In view of this, it has been attempted to reduce the concentration of oxygen in an atmosphere in which R—Fe—B alloy powder is produced to suppress generation of the oxide R2O3, and to thereby reduce the relative amount of the R in the finally manufactured R—Fe—B rare earth magnet and thus improve the magnetic properties of the magnet.
The amount of oxygen in R—Fe—B alloy powder used for manufacture of an R—Fe—B magnet should preferably be small, as described above. However, no attempt to reduce the amount of oxygen in R—Fe—B alloy powder for improving the magnet properties has been realized as a mass production technique, for the following reason. If R—Fe—B alloy powder is produced in a controlled environment of a low oxygen concentration so that the amount of oxygen in the alloy powder is as low as 4000 wt. ppm or less, for example, the powder may vigorously react with oxygen in the atmosphere (the air), causing the possibility of ignition in several minutes at room temperature.
Hydrogen processing for milling provides good production efficiency compared with mechanical milling using a ball mill, for example. However, when magnet powder produced by the hydrogen processing is used for manufacture of a magnet, the resultant magnet tends to vary in magnetic properties (coercive force among others) depending on the sintering conditions. In particular, the variation in magnetic properties is significant when the amount of oxygen in the sintered body is as small as 4000 wt. ppm or less and the total amount of the rare earth element in the magnet is comparatively small (e.g., 32 wt. % or less).
Therefore, while it has been recognized that the amount of oxygen in R—Fe—B alloy powder should desirably be reduced for improving the magnetic properties, in reality, it is extremely difficult to handle R—Fe—B alloy powder having a reduced oxygen concentration in a production site such as a plant.
In particular, the risk of ignition is high during a pressing or compacting process in which powder is compacted with a press. In this process, the temperature of a compact rises due to heat generated as the result of friction among powder particles during compaction and as a result of friction between powder particles and the inner sidewall of a cavity of the press during ejection of the compact. One possible technique for prevention of ignition includes placement of the press in an environment of a non-oxygen atmosphere. This placement is however impractical because supply of the raw material to the press and retrieval of the compact from the press are difficult in such a non-oxygen environment. The occurrence of ignition may also be avoided if individual compacts are immediately sintered when they are ejected from the press. This is, however, an extremely inefficient process, and thus not suitable for mass production. A sintering process takes four hours or more, and it is reasonable that each sintering process is carried out against a lot of compacts at the same time. In addition, in mass production facilities, it is difficult to manage compacts in an environment of an extremely low oxygen concentration through a series of processing steps from pressing to sintering.
A liquid lubricant such as fatty ester is often added to fine powder before the pressing process to improve compressibility or formability of the powder. By this addition of a liquid lubricant, thin oily coatings are formed on the surfaces of powder particles. Such coatings however fail to sufficiently prevent oxidation of the powder having an oxygen concentration of 4000 wt. ppm or less.
For the above reasons, a slight amount of oxygen is intentionally introduced into an atmosphere in which an R—Fe—B alloy is milled, to thereby oxidize thin surfaces of finely milled powder particles and thus reduce the reactivity of the powder. In an example of such a technique, Japanese Patent Publication No. 6-6728 discloses a process in which a rare earth alloy is finely milled under a supersonic inert gas flow containing a predetermined amount of oxygen, so that during the milling a thin oxide coating is formed on the surfaces of fine powder particles produced by the milling. According to this technique, since oxygen in the atmosphere is blocked by the oxide coatings on the powder particles, occurrence of heat generation/ignition due to oxidation is prevented. Note, however, that with the existence of the oxide coatings on the surfaces of the powder particles, the amount of oxygen contained in the powder increases.
U.S. Pat. No. 5,489,343 and Japanese Laid-Open Patent Publication No. 10-321451 disclose another technique where R—Fe—B alloy powder having a low oxygen content (for example, 1500 ppm) is mixed with mineral oil or the like to obtain slurry. Since powder particles in the slurry are kept from contact with the atmosphere, occurrence of heat generation/ignition is prevented while the oxygen content of the R—Fe—B alloy powder is kept low.
This conventional technique has the problem that after the R—Fe—B alloy powder in the slurry state is filled in a cavity of a press, the oil must be squeezed out during the pressing process. This reduces productivity. Further, conventional methods for manufacturing a rare earth magnet have the problem that crystal grains tend to become coarse during sintering. The magnet properties (coercive force) therefore fail to be improved sufficiently even when magnet powder having a low oxygen concentration is used.